Flatten a Curved Section of a Mesh
You can do this fairly easily if you use proportional editing and scaling.

Turn on proportional editing:


Select the top layer of vertices using box select:


Scale (S) along the Z axis (z) then press 0 to scale it it flat. Scroll your mouse wheel to adjust how much of the mesh it affects:


And you get:



If your curve isn't flat like in the previous example, it's still possible, it just takes a little more work.

Select the edge loops around the area you want to flatten (using Alt Right click):


Pull them out to be flat with the middle edge loop.

Shift-Select the middle edge loop and push all of them in:


Continue tweaking it until it's flat enough: